Field of Art
This disclosure generally relates to fiber based optical parametric oscillators.
Description of the Related Art
Light sources based on optical parametric interaction are of significant interest because they provide access to laser wavelengths that existing gain materials based on electronic transitions cannot provide. An optical parametric oscillator (OPO) can be realized by exploiting the χ(2) nonlinear optical response in a wide range of crystals or the χ(3) nonlinear response in optical fibers.
Optical fiber based OPO (FOPO) are particularly attractive owning to their potential in achieving low cost, alignment-free and compact laser systems while still providing very wide tuning range and high power operation.
The operation of FOPOs is in essence based on degenerated four-wave-mixing (FWM) wherein two pump photons interact with the fiber to generate a signal photon and an idler photon. The exact frequencies of the signal and idler photons are defined by the phase matching condition which depends on the pump laser wavelength, its peak power as well as the dispersion profile of the optical fiber.
In the prior art, a wavelength-swept FOPO has been constructed which includes a wavelength swept pump laser. The wavelength swept pump laser was programmed to generate a linear sweep of the pump wavelength. The pump wave was then amplified by a first erbium-doped fiber amplifier (EDFA). Next, a Mach-Zehnder modulator (MZM) was used to generate pulses with low duty ratio. The pump pulses were then amplified by a second EDFA which acted as a pre-amplifier. The pre-amplifier was then followed by a third EDFA which acted as a booster amplifier. A high speed voltage-controlled tunable Fabry-Perot filter (FP) which was controlled by an arbitrary wavelength generator (AWG), was inserted between the pre-amplifier and the booster amplifier to reject the amplified spontaneous emissions (ASE) from the pre-amplified and thus mitigated the gain tilt in the booster amplifier. The AWG was synchronized with the wavelength swept pump laser such that output of the booster amplifier was a high power fiber coupled swept laser.
The output of the booster amplifier was then injected into a FOPO ring. This was done by combining the output of the booster amplifier with the lasing light of the FOPO ring at 1.3 μm through a first wavelength division multiplexer WDM coupler. The parametric gain medium was provided by a 100 meter (m) spool of Dispersion shifted fiber (DSF) with a zero-dispersion wavelength (ZDW) of 1565.7 nm and dispersion slope of 0.07 ps/nm/km. The DSF was followed by a first 1% tap, the 1% port of the first 1% tap was used monitor the DSF output. The 99% port of the first 1% tap was fed into a second WDM coupler. One port of the second WDM coupler was used to dump the residual pump and a 1.9 μm idler. A second port of the second WDM coupler was coupled into an Optical Delay Line (ODL) which was used to adjust the length of the FOPO ring resonating cavity. A polarization controller was placed after the ODL to control the polarization state of the FOPO. A second 1% tap was placed after the polarization controller. The 1% port of the second 1% tap was used to characterize the lasing wave output of the FOPO. The 99% port of the second 1% tap was fed into the first WDM coupler thus completing the FOPO ring resonating cavity.
As a pump light which has a high peak power is coupled into the parametric gain medium in the FOPO cavity, self-phase-modulation (SPM) occurs in the fiber which connects the gain fiber and the parametric gain medium. As a result, spectrum bandwidth of the pump light is broadened, and the conversion efficiency from pump light to signal light is decreased. Thus, it becomes more difficult to achieve high peak power of signal light.
FIG. 1 is an illustration of a typical FOPO light source 100 in which the distance the distance between the linear gain medium and the parametric gain medium is highlighted. A light source 100 is pumped by a seed laser 102. The seed laser 102 is spliced to a first polarization controller 106a. The first polarization controller 106a is spliced to a C port of a first wavelength division multiplexer (WDM) coupler 108a. The first WDM coupler 108a may be a C/L band fused fiber coupler. The multiplexed port of the first WDM coupler 108a is spliced to a signal port of a second WDM coupler 108b. The second WDM coupler 108b may be a 980/1550 fused fiber coupler. A first 980 nm pump laser 110a is connected to a pump port of the second WDM coupler 108b. The multiplexed port of the fused fiber coupler is coupled to an Er-doped fiber (EDF) 112. The EDF 112 is spliced to a multiplexed port of a third WDM coupler 108c. The third WDM coupler 108c may be a 980/1550 fused fiber coupler. A second 980 nm pump laser 110b is connected to a pump port of the third fused fiber coupler 108c. The pulse train from the seed laser is amplified by an Er-doped fiber amplifier (EDFA), which is described above is located inside the FOPO cavity.
The output of the EDFA exits the signal port of the third WDM 108c, which is spliced to dispersion shifted fiber (DSF) 114 which is used as the parametric gain medium. The output of the DSF 114 is spliced to an input port of a power splitter 116. The power splitter 116 may be a fused fiber coupler. The power splitter may have a 90%/10% split. The 90% output port may produce the output signal 104. The 10% output port may be spliced to an optical delay line (ODL) 118. The cavity length may be adjusted within a few centimeters using the fiber-coupled ODL 118. The output of the ODL 118 may be spliced to a polarization controller 106b which may then be spliced to an L port of the first WDM coupler 108a, thus completing a resonant ring cavity for the FOPO.
The typical manufacturing process for fiber optic systems such as light source 100 which includes specialty fiber (DSF and EDF) is to pigtail the specialty fiber. This pigtailing process usually includes attaching standard fiber such as SMF-28 or an equivalent to the specialty fiber with a fusion splicer. A typical fusion splicer will include recipes for splicing standard fiber to standard fiber and splicing common specialty fibers to standard fiber. Recipes for splicing specialty fiber to standard fiber take into account the differing mode field diameters of the two fibers by forming a taper on one side of the splice. The recipes also take into account the different material properties and structure of the fibers.
The pigtailing process usually includes adding 0.5-1 meters of standard fiber to the specialty fiber. This allows for greater control over the performance of the device, by allowing active monitoring of the splice, separation of non-standard splicing workflow from the standard fiber splicing workflow, and allows for additional freedom in the placing of the splices, fiber optic components and gain fiber so as to minimize splice loss.